This happened to me too, a few months back.
Here’s what I figured out at the time.
Western Union subscribes to a service called Cyveillance. Cyveillance is a bot that searches the web for trademarked words and phrases, or whatever Western Union (or whomever) wants it to search for. When the bot finds a match, something happens — e.g. Cyveillance might send a report to WU, and then WU sends the warning letter out, or it might be set up so that Cyveillance takes care of everything.
Exactly how it works, I’m not sure…. What I do know, is that blocking the Cyveillance spider might prevent this from happening again. I don’t know what the IP range is for Cyveillance, but I’m sure you could find it by doing a quick G search.